The number of mouse strains used in biomedical research is growing rapidly as a result of the technology that allows one to selectively alter mammalian genomes. To insure rapid and efficient advancement of biomedical research, it is vitally important that these new transgenic, targeted mutant, and chemically mutagenized mice, along with other equally valuable mouse strains, be protected against accidental loss. Cryopreservation of embryos or sperm offers an economical means of preserving scientifically important strains for future biomedical research. TJL has established the largest repository for frozen germplasm of genetically defined strains and has over 25 years of experience in cryopreservation. The objective of this project is to assure the continuity of this repository and the availability of mice from preserved germplasm to the scientific community. The Specific Aims of this application are to: 1) maintain the infrastructure of the continually growing TJL repository; 2) recover and distribute mice from frozen germplasm as needed for scientific research; 3) cryopreserve germplasm from scientifically valuable strains of mice that are not supported by other specific funding sources; 4) carry out research to develop, refine, and/or acquire new methods to improve the efficiency and productivity of the cryopreservation resource; and 5) interact and coordinate efforts with other resource centers.